The present invention relates to electric meters and, more particularly, to apparatus for adjusting the phase lag of a voltage stator of an electric meter.
Conventional electro-mechanical electric meters employ a conductive metal disk rotated as the rotor of a small induction motor by interaction with fluxes generated by opposed voltage and current coils or stators. When the fluxes produced by the current and voltage stators are in quadrature, the rotational torque experienced by the disk is proportional to the voltage applied to the load multiplied by the current consumed by the load; that is, the power consumed by the load. Disk rotation is magnetically resisted in proportion to its rotational speed. Thus, the disk speed is proportional to the power consumed by the load. Each rotation of the disk represents a predetermined increment of energy consumed. The rotations of the disk are accumulated over time in a mechanical or electronic accumulator, or register, for billing purposes by the utility supplying the power.
The voltage stator conventionally includes a large number of turns of fine wire on a laminated core of silicon steel. Resistive losses in the windings and the core of the voltage stator tend to shift the phase angle of the flux developed by the voltage stator to a value different from the desired 90 degrees relative to the phase angle of the flux developed by the current stator. In addition, the reaction of the disk to flux produced by the voltage stator may differ from the reaction to flux produced by the current. Furthermore, resistive losses and disk reaction tend to shift the phase angle of the flux developed by the current stator.
It is conventional to provide a means for adjusting the phase angle of the flux developed by the voltage stator to produce the desired disk speed at rated load at a power factor of 0.5. One common method of performing this adjustment employs a conductive metallic plate in series with a one-turn loop through which the flux from the voltage stator must pass. The metallic plate includes a number of closely spaced holes separated by narrow webs. During final adjustment of the meter, some of the webs are cut apart to increase the path length of current flow through the metallic plate, and thereby increasing the resistance thereof.
The above adjustment technique suffers from several drawbacks. First, the adjustment is one which is necessarily performed during final factory adjustment and is not repeatable in the field. Second, the adjustment technique is labor intensive since a substantial time is consumed in testing and then cutting the webs. Third, only step adjustments are possible. Practically, only about 10 to 15 step adjustments are feasible. If a speed change per step of, for example, about 0.5 percent is desired, a total adjustment range of from about 5 to about 7.5 percent is possible. In some applications, it is desirable to provide both finer adjustment than 0.5 percent and greater range than 7.5 percent. Finally, once a web has been cut, there is no going back. That is, if the meter speed is over-adjusted by the cutting of one or more webs in addition to those whose cutting is necessary to achieve the desired regulation, there is no practical method for backing off on the adjustment. Once a web is cut it remains cut, and the only alternative includes disassembly of the meter and replacement of the conductive plate with one which does not have cut webs. The adjustment procedure must then be repeated.
Other adjustment techniques include overcompensating the phase angle of the voltage stator and using a slide-wire variable resistance adjustment on the current stator. The need for doubling the amount of phase lag adjustment hardware entailed by this technique makes it less than ideal.